The preparation of aromatic polycarbonates by various processes is well known. Until recently, however, when using phosgene to provide the carbonate linkage, a substantial excess of the calculated stoichiometric amount was added to the reactor vessel to insure that a polycarbonate of the appropriate degree of polymerization was actually prepared.
U.S. Pat. No. 4,378,454 issued Mar. 29, 1983, discloses a method for determining the end point of the polycarbonate polymerization reaction. This method is based on the known solubility of phosgene in the same liquid that polycarbonate resin is soluble. Phosgene is essentially instantaneously soluble in solvents such as halogenated organics, e.g., methylene chloride and the like. Once solubilized the phosgene reacts with the bisphenol or active bisphenol derivative, for example sodium bisphenate. Since there is insufficient bisphenol or active bisphenol derivative to react with the phosgene solubilized in the organic solvent, once the stoichiometric end point has been reached, the additional solubilized phosgene can now be detected by the known phosgene color test described in U.S. Pat. No. 4,378,454.
It has now been discovered that at or slightly after the time the stoichiometric end point of the aromatic polycarbonate preparation has been reached, a substantial increase in phosgene concentration in the overhead portion of the reactor, that is, the vapor phase, also occurs. This increase in phosgene concentration can be detected by any standard means and signals the end point of the polymerization reaction. Phosgene addition can then be terminated, thereby saving the extra phosgene which would have been added to insure the achievement of the polymerization end point. Reducing the amount of phosgene also reduces the amount of time for each polymerization, whether batch or continuous, thus increasing the effective capacity of present plant equipment.